Overexposure to sunlight, which is damaging to natural photosynthetic systems of green plants and cyanobacteria, is also expected to be damaging to artificial photosynthetic systems. Nature has solved the problem through a photoprotection mechanism called “nonphotochemical-quenching,” in which excess solar energy is safely dissipated as heat from one molecular system to another. With an eye on learning from nature’s success, a team of Berkeley Lab researchers has discovered a surprising key event in this energy-quenching process.

Translocation of the carotenoid pigment within a critical light-sensitive protein called the Orange Carotenoid Protein triggers a shifting of the protein from the light-absorbing orange state to the energy- quenching red state, providing cyanobacteria with protection from too much sunlight.

In a study led by Cheryl Kerfeld, a structural biologist who holds joint appointments with Berkeley Lab’s Physical Biosciences Division and Michigan State University, the research team found that in cyanobacteria the energy-quenching mechanism is triggered by an unprecedented, large-scale movement (relatively speaking) from one location to another of the carotenoid pigment within a critical light-sensitive protein called the Orange Carotenoid Protein (OCP). As a result of this translocation, the carotenoid changes its shape slightly and interacts with a different set of amino acid neighbors causing the protein to shift from an “orange” light-absorbing state to a “red” photoprotective state. This turns out to be an unanticipated molecular priming event in photoprotection.

“Prior to our work, the assumption was that carotenoids are static, held in place by the protein scaffold,” Kerfeld says. “Having shown that the translocation of carotenoid within the protein is a functional trigger for photoprotection, scientists will need to revisit other carotenoid-binding protein complexes to see if translocation could play a role in those as well. Understanding the dynamic function of carotenoids should be useful for the design of future artificial photosynthetic systems.”